Yz450f won't start :(

By SH3A, Posted November 23, 2015

I recently rebuilt my 09 yz and I put new rings, valves, seals, cylinder and gaskets in it, I lapped the valves seats with the old valves (is this ok, it wasn't on purpose it was on purpose the first time as I wanted to properly seat the valves but then grey racer told me that is a no go so I replaced the valves.) I haven't gotten the head reseated and don't have the money for it either. I wasn't planning on spending $590 on a new head and cylinders originally.... So that hurt my wallet but now hoping I don't have to spend to much more $$$$, so back to the original question, after the 2nd rebuild I put fresh oil and filter in and tried to start it and it fired up in like 30 kicks with ALOT of backfires. Let it idle for 5 min. Went to start it today (5days later) I can't get it to start 50 kicks and a backfire every-other kick. No what?

By kawasaki_873, Posted November 23, 2015

titanium valves or SS? anytime valves are replaced the new seats NEED to be cut regardless. lapping should only be done with SS valves not titanium. did you get the timing right? what was your measured valve clearances after the rebuild?

By SH3A, Posted November 24, 2015

titanium valves or SS? anytime valves are replaced the new seats NEED to be cut regardless. lapping should only be done with SS valves not titanium. did you get the timing right? what was your measured valve clearances after the rebuild?

I'll check clearances again tommorow but I re shimmed after the new valves to the appropriate value, I'm sure timing is correct as it was running and idling fine 5 days ago, I don't believe timing can just snap out of place"well hopefully"